typedef struct ListNode ListNode;
int kthToLast(struct ListNode* head, int k)
{
    ListNode* fast = head, *slow = head;
    //快指针先走k步
    while(k--)
    {
        fast = fast->next;
    }
    //同时走
    while(fast)
    {
        slow = slow->next;
        fast = fast->next;
    }
    return slow->val;
}